The horse population on Shackleford Banks, a barrier island off the coast of North Carolina, has been controlled using a program of porcine zona pellucida immunocontraception since 2000. Prior studies found no negative physiological effects of the immunization, but suggested some adverse effects on behavior such as increased harem switching and extended reproductive season. Although no mares were immunized in the year of study (2013), we examined the effects of past contraception and foaling on health and behavior of the mares in an attempt to better understand its impacts on both the individual and the population. Current lactation was found to be the best predictor of health and behavior: lactating mares were in worse physical condition, spent more time grazing, and switched harems fewer times. Mares that had never foaled also were in better physical condition than mares that had foaled in the past, but did not differ in behavioral measures. There were indications that the PZP treatment has had effects other than preventing pregnancy: an increased number of years since immunization was correlated with better body condition and fewer harem switches, independent of lactation. Contraception treatment intensity or timing did not have any significant adverse effects on health and behavior on its own. However, three or more consecutive years of treatment or administration of the first dose before sexual maturity may have triggered infertility in some mares. Inducing sterility, while relieving the mares from the energetic costs of lactation and reducing the stress from harem switching, may have unintended consequences on population dynamics by increasing longevity and eliminating the mares’ ability to contribute genetically.